Vue - Deep watching an array of objects and calculating the change?

Your comparison function between old value and new value is having some issue. It is better not to complicate things so much, as it will increase your debugging effort later. You should keep it simple.

The best way is to create a person-component and watch every person separately inside its own component, as shown below:

<person-component :person="person" v-for="person in people"></person-component>

Please find below a working example for watching inside person component. If you want to handle it on parent side, you may use $emit to send an event upwards, containing the id of modified person.

Vue.component('person-component', {
    props: ["person"],
    template: `
        <div class="person">
            {{person.name}}
            <input type='text' v-model='person.age'/>
        </div>`,
    watch: {
        person: {
            handler: function(newValue) {
                console.log("Person with ID:" + newValue.id + " modified")
                console.log("New age: " + newValue.age)
            },
            deep: true
        }
    }
});

new Vue({
    el: '#app',
    data: {
        people: [
          {id: 0, name: 'Bob', age: 27},
          {id: 1, name: 'Frank', age: 32},
          {id: 2, name: 'Joe', age: 38}
        ]
    }
});

<script src="https://unpkg.com/[email protected]/dist/vue.js"></script>
<body>
    <div id="app">
        <p>List of people:</p>
        <person-component :person="person" v-for="person in people"></person-component>
    </div>
</body>

It is well defined behaviour. You cannot get the old value for a mutated object. That's because both the newVal and oldVal refer to the same object. Vue will not keep an old copy of an object that you mutated.

Had you replaced the object with another one, Vue would have provided you with correct references.

Read the Note section in the docs. (vm.$watch)

More on this here and here.

I have changed the implementation of it to get your problem solved, I made an object to track the old changes and compare it with that. You can use it to solve your issue.

Here I created a method, in which the old value will be stored in a separate variable and, which then will be used in a watch.

new Vue({
  methods: {
    setValue: function() {
      this.$data.oldPeople = _.cloneDeep(this.$data.people);
    },
  },
  mounted() {
    this.setValue();
  },
  el: '#app',
  data: {
    people: [
      {id: 0, name: 'Bob', age: 27},
      {id: 1, name: 'Frank', age: 32},
      {id: 2, name: 'Joe', age: 38}
    ],
    oldPeople: []
  },
  watch: {
    people: {
      handler: function (after, before) {
        // Return the object that changed
        var vm = this;
        let changed = after.filter( function( p, idx ) {
          return Object.keys(p).some( function( prop ) {
            return p[prop] !== vm.$data.oldPeople[idx][prop];
          })
        })
        // Log it
        vm.setValue();
        console.log(changed)
      },
      deep: true,
    }
  }
})

See the updated codepen

Vue - Deep watching an array of objects and calculating the change?

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